The major objective of this study is to evaluate thymic transplants as a means of reconstitution of the immune system in SIV-infected, immunosuppressed pig-tailed macaque monkeys. This will include an evaluation of both allogeneic and xenogeneic transplants. Studies will entail use of an SIV that results in a decrease of CD4+ cells to 200/cumm, or lower, within 3 to 4 months, and will employ the use of antivirals (PMPA) to control virus load prior to the time of thymic transplant. An initial group of pig-tailed macaques has been inoculated with SIVsmmFGb. One of these animals died 2 months post-infection as a result of severe CNS disease associated with the SIV infection, and three additional animals have CD4+ cell counts of 200/cumm or less at 3 months post-infection. These animals are currently being treated with PMPA and will receive thymus transplants about 3 weeks after the initiation of PMPA treatment. Observations in these studies should be applicable to efforts toward immune reconstitution in HIV-infected, immunosuppressed individuals.